The overall goal of this project is to understand the mechanisms that control the growth, morphogenesis and differentiation of the ureteric bud during kidney development. While it is established that GDNF, signaling through the receptor tyrosine kinase Ret and the co-receptor GFRalpha1, plays an essential role in this process, the downstream mechanisms remain largely obscure. We hypothesize that GDNF regulates the expression of a set of target genes, which in turn mediate the different functions of the developing ureteric bud, including growth, branching morphogenesis, differentiation into specific collecting duct cell types, and production of factors that induce the mesenchymal stem cells to undergo nephrogenesis. In Aim 1 we propose to identify the target genes whose expression in the ureteric bud is induced or repressed by GDNF. Our preliminary data indicate that the screen will be highly effective and will allow us to identify a variety of genes with important and unforeseen roles in kidney development. In Aim 2 we propose to conduct several modified screens to investigate the mechanisms by which GDNF regulates the expression of its target genes, via different isoforms of the Ret receptor and different intracellular signaling pathways. In Aim 3 we propose a series of functional studies in mouse models to investigate the roles of several novel GDNF target genes in renal development. The initial target genes to be examined will include a chemokine receptor that may play a role in the regulation ofureteric bud branching, a secreted cytokine that may be involved in the induction of nephrogenesis, and a transcription factor that could mediate regulation of secondary target genes. The proposed methods include loss-of-function studies using knockout and other mutant mice, gain-of-function studies using transgenic mice, as well as in vitro organ culture experiments. In addition to being a fascinating problem in developmental biology, the control of ureteric bud morphogenesis has important clinical implications, as renal agenesis, ureteral defects and renal hypodysplasia are common birth defects that can of Len be attributed to defects in the development of the ureteric bud.